The invention relates to the field of projectors for projecting picture slides or diapositives onto a viewing screen, and particularly to such projectors which have means for so-called automatic focusing. Automatic focusing, in this sense, refers to mechanism for keeping the projection lens and the central portion of the picture slide or diapositive at a constant distance from each other, notwithstanding the fact that, under the influence of heat from the projection lamp, the picture slide may buckle while it is in the projection position or projection gate. Of course the automatic focusing mechanism does not really focus the image on the screen automatically, and this must be done by the operator when the projector and screen are initially set up. But after the projection lens has been adjusted to a sharp focus position, depending on the distance of the viewing screen from the projector, the automatic focusing mechanism will thereafter be effective to keep the image on the screen in sharp focus, notwithstanding possible buckling of the picture slide while it is being projected.
Various different kinds of projectors of this general type are known in the art. They are often used in pairs, set side by side and projecting on the same screen, and are controlled, either manually or by a known form of automatic control, in such manner that while a picture is being projected on to the screen by one projector, the slide is being changed in the other projector, and then the picture from the first projector fades out while the picture from the newly inserted slide in the second projector fades in. The present invention relates to an improved form of fading mechanism, utilizing the focusing motor to shift the projection lens to a nonfocused or fading position, preferably with simultaneous dimming of the projection lamp, and utilizing this same focusing motor plus other portions of the automatic focusing mechanism to bring the projection lens back to the proper sharp focused position, simultaneously brightening the projection lamp if it has previously been dimmed. With the present invention, no additional motor is needed to produce the fade-out or blurred focusing effect.
Although the invention is particularly useful when employing twin projectors as above mentioned, it is also useful when only a single projector is used. The present invention enables the changing of slides in a visually pleasing manner, without excessive brightness fluctuations on the viewing screen when a slide is being changed.